The conventional practice of removably mounting blowout preventers upon casing heads so as to prevent blowouts during drilling operations requires that: the casing head have a lower bore substantially the same as the casing upon which it is mounted; the casing head have an enlarged upper bore to support a hanger for the next string of casing; that the drilling assembly temporarily mounted above the casing head have a bore at least as large as the upper enlarged bore of the casing so as to pass the casing hanger therethrough. Because the lower bore of the casing head is smaller than the bores above, a tapered shoulder known as a bowl connects the lower casing head bore and the enlarged upper bore of the casing head, the bowl being subject to damage by drill string members passing through the casing head bore. The bowl is used for support of a casing hanger required to suspend the next string of casing and is used for sealing the annulus around the next string of casing, in cooperation with the casing hanger. Therefore, it is most important that the bowl be protected from damage because a failure of the bowl to support and seal, can and has caused expensive and dangerous remedial action, and in some cases has caused blowouts, loss of the well and even loss of life.
Various devices have been used heretofore to guide the drill string members, such as retractable rams mounted within an additional head mounted above the casing head. However, because of the extra space requirements, the extra costs, and the extra connections that may leak, other alternatives have been tried that fit into the upper enlarged bore of the casing head, the rings having relatively small vertical slots around their periphery so as to allow for the passage of returning drilling mud. Such small slots however, result in too high a velocity of upwardly flowing drilling mud returning from the well bore as when the rig is substantially blocked when passing a tool joint. Because the returning drilling mud contains extremely hard and abrasive particles cut from the formation being drilled and because it is flowing at high velocity through the small slots of the ring, the bowl often becomes cut such that it is rendered useless for sealing or for support purposes.
Applicant has found that: by providing an inner ring to guide the drill string members inwardly so as to prevent their contact with the bowl; by providing lugs around the ring for radial and vertical support of the ring against the bowl; by making the accumulative width of the lugs around the ring less than the accumulative width of the slots around the ring; that mud flow passages are formed large enough so as to reduce the mud velocity sufficiently to substantially eliminate the cutting of the bowl by the abrasive particles and to thereby preserve the sealing and support capability of the bowl.
Therefore, objects of the present invention are to: guide drill string members so as to prevent their contact with the bowl; prevent mechanical damage to the bowl; prevent cutting action of the bowl caused by high velocity flow of abrasive particles; preserve the sealing integrity of the bowl; preserve the support capability of the bowl; prevent replacement of expensive equipment; save rig time by preventing equipment replacement; prevent blowouts by maintaining sealing and support capabilities of the bowl.
These and other uses and advantages will become obvious to those skilled in the art upon practice of the present invention.